Thunderbird add-ons that I've either used for years, or tested and highly suggest.
Quicktext is an extension for Thunderbird that lets you create templates that can be easily inserted into your own emails.
Displays the address books in a sidebar in Thunderbirds 3-pane-window and can be toggled with the F4 key or a toolbar button.
Visit my Contacts Sidebar site for recent versions and news.
Add symbol in header plane to switch between compact and expanded view. Options to switch between one or two lines compact view, which buttons should be displayed in header pane. Linkify RSS subject. Optionally darkens header if message has focus.
Reinstates the extensions.checkCompatibility preference without respect to the current application version.
Extracts all attachments from selected messages and then can delete, detach or mark-read.
The "Swiss army knife" of folders. Turn your favorites into tabs - open folders and subfolders, move mails without scrolling around or searching. powerful + easy.
See new mail at a glance after automatic filtering - supports quickFilters addon!
Synchronizes selected addressbooks with other instances of thunderbird
using ftp/webdav/imap servers
Add more items to Add-ons Manager context menu.
Allows for customization of the folder pane. Accounts can be rearranged and the startup folder can be chosen.
Check the message contents and addresses before you send the message...
Thunderbird Message Filter Import/Export is a thunderbird/mozilla extension to import/export message filters.
Displays icon for user agent of received mails.
Associates user-defined colors with accounts and identities. Applies these colors to accounts, identities, folders, message list, message headers and compose headers. Users will always know which accounts/identities they are working with.